The invention relates to a metal spinning machine in accordance with the introductory part of claim 1.
Metal spinning machines and their corresponding roller holders are known and have long been used for the cold forming of rotationally symmetrical workpieces. The roller holders serve especially for a rapid changeover of the machine from one roller to another. In modern metal spinning machines a plurality of roller holders are usually held in a magazine and can be selectively fastened to the tool rest, and then the selection and mounting as well as removal of the roller holder takes place automatically the same as the spinning process, under the control of a program.
A widely used method for the production of control data for the program is the so-called "playback" method in which first the metal spinning machine is operated by a workman and at the same time the individual steps of the operation and the roller positions they produce are electronically detected and recorded as data. With these recorded data the same working procedure is then repeated as often as desired under the controlling program, with precision and great speed, while the roller successively runs through the recorded positions.
Especially in the case of workpieces of a soft material such as aluminum or copper, when fluctuations or irregularities in the thickness of the blank workpieces occur, operating defects can occur which in the most unfavorable case can result in the parting of the workpiece as a result of excessive tool pressure. To prevent this it is necessary to limit the force exerted by the roller on the workpiece.
European Patent 0 125 720 B1 discloses a metal spinning machine provided with such a force limiting system. In this machine the roller holder is made rigid, but a spring unit is disposed between the top rest of the tool rest unit and the system driving the top rest. While the spinning is in progress, a sensor detects the force exerted through the roller holder and the top support on its drive system, and the force that is detected is substantially the same as the force exerted by the roller on the workpiece. The detected force values are fed to a control unit and under the control of the program the carriage bearing the roller holder is backed off by means of its drive unit if a preset degree of compression of the spring unit is exceeded, or if the said compression is less than the preset degree.
It is considered to be a disadvantage of this method that its technical complexity is very great, because a special top rest with spring unit and sensor is needed, and an expansion of the control program is necessary. Also, there is the disadvantage that the parts to be moved, namely the top support carriage, the roller holder and the roller together constitute a great mass, which results in a relatively low speed of reaction. Since this furthermore involves a control system, an oscillation or resonance can occur in the positioning of the roller, which can be prevented only by damping which reduces the speed of reaction. This machine, therefore, is hardly suitable for fast machining operations.
The problem therefore arises of creating a metal spinning machine of the kind described above, in which an effective and fast-reacting limitation of the force exerted by the roller on the workpiece can be achieved without great complexity and cost, and without the occurrence of oscillation or hunting.
This problem is solved according to the invention by a metal spinning machine of the kind described above, with the distinctive features of claim 1.
With the metal spinning machine according to the invention it is brought about that the force that can be exercised by the roller on the workpiece can be limited in a simple manner to harmless levels by setting an appropriate pressure of the pressure medium. Furthermore, by the use of the piston-and-cylinder arrangement, the force acting on the piston can be kept constant over the entire stroke, i.e., a largely linear characteristic is achieved. Since the moving masses are very small, a very high speed of reaction is assured, which permits a high operating speed. Furthermore, no control system is needed here, which eliminates oscillation and hunting, and renders sensors and other additional control and regulating devices superfluous. The technical cost therefore remains limited, added to the fact that only the roller holder of the spinning machine has to be modified, while all the rest of the parts of the metal spinning machine as well as the control program can be left completely unaltered. Consequently the invention is also suitable for the upgrading of existing metal spinning machines, because only the roller holder needs to be replaced. A compressor or hydraulic pump, for example, is usually already present anyway, so that, aside from any pressure regulating unit that might be necessary, no great additional cost is involved. With the new metal spinning machine, therefore, improved product quality is achieved, without involving great additional cost for the machinery, and without requiring additional working procedures or additional time.
Preferred configurations and further developments of the invention are set forth in the secondary claims.